Lightweight portable postal scales serve a useful function for postal patrons who do not have convenient access to more complicated platform scales available at post offices and elsewhere. In particular, a portable postal scale enables the user to determine whether the weight of a letter exceeds a maximum value such as one ounce.
Various types of portable postal scales are described in prior art patents. U.S. Pat. No. 2,284,526 shows a scale having a beam formed of a thin strip of metal with a thicker counterweight arm secured to one end of the beam. A clip for clamping a letter is disposed at the other end of the beam, and a downward-projecting transverse fulcrum is provided at an intermediate location on the beam by the downwardly bent end of the counterweight arm. In operation the scale is placed on a flat surface with the clamping end extending over its edge so that the weight of the letter may cause the beam to pivot downward if a predetermined weight is exceeded. Another type of scale using a balance beam and a downward-projecting fulcrum is disclosed in U.S. Pat. No. 2,949,287. This scale requires placement of a letter flatwise on an arm of the beam with its center of gravity at a predetermined distance from the fulcrum. U.S. Pat. No. 3,820,322 also discloses a device having a flat beam and a downward-projecting transverse fulcrum, the letter to be weighed being placed with its center of gravity on the longitudinal axis of the beam. Other approaches are shown by U.S. Pat. Nos. 3,224,516 and 4,053,024, which are respectively concerned with use of a magnetic fulcrum pin insertable in transverse grooves and with a flexible planar strip of material bent back on itself to provide a spring effect.
The prior art scales discussed above present various limitations and disadvantages. The beam devices that use a downward-projecting fulcrum are not amenable to performance of an additional function, that is, measurement of the thickness of a letter by attempting to slide it under the clamp holding end of the beam. A downward-projecting fulcrum rendered such measurement inaccurate. Devices that require placement of a letter with its center of gravity at a predetermined location introduce inaccuracy to the weighing function because of variances in the center of gravity from one letter to another with equal outside dimensions. In addition, the prior art devices employ a relatively flat beam with little or no space on the beam sides for printed material other than necessary for their weighing function. A thicker beam in a form such as a generally rectangular block would provide space on its sides for advertising material or the like and enable marketing of the scale as an advertising novelty device.